Monocotyledon

Hemerocallis flower, with three flower parts in each whorl

Wheat, an economically important monocot

The Monocotyledons or monocots are a group of flowering plants, dominating great parts of the earth. Monocots comprise the majority of agricultural plants in terms of biomass produced. There are between 50,000 and 60,000 species within this group; according to IUCN there are 59,300 species.

The largest family in this group (and in the flowering plants) is the orchids (usually taken to be the family Orchidaceae, but sometimes treated at the rank of order), with about twenty thousand species. These have very complex (and striking) flowers, adapted for highly specific insect pollination.

The economically most important family in this group (and in the flowering plants) are the grasses, family Poaceae (Gramineae). These include all the true grains (rice, wheat, maize, etc), the pasture grasses and the bamboos. This family of the true grasses have evolved in another direction, becoming highly specialized for wind pollination. Grasses produce much smaller flowers, which are gathered in highly visible plumes (inflorescences). A further noteworthy, and economically important, family is the palm family Arecaceae (Palmae).

Name, characters

Grass sprouting on left (a monocot), showing a single cotyledon. Compared to a dicot (right)

The name monocotyledons is derived from the traditional botanical name Monocotyledones, which derives from the fact that most members of this group have one cotyledon, or embryonic leaf, in their seeds. This as opposed to the traditional Dicotyledones, which typically have two cotyledons. From a diagnostic point of view the number of cotyledons is neither a particularly handy (as they are only present for a very short period in a plant's life), nor totally reliable character.

Nevertheless, monocots are a distinctive group. One of the most noticeable traits is that a monocot's flower is trimerous, with the flower parts in threes or in multiples of three. For example, a monocotyledon's flower typically has three, six, or nine petals. Many monocots also have leaves with parallel veins.

Hypoxis decumbens L. with a typical monocot perigone and parallel leaf venation

Morphology, compared to the (former) dicotyledons

The traditionally listed differences between monocotyledons and dicotyledons are as follows. This is a broad sketch only, not invariably applicable, as there are a number of exceptions. The differences indicated are more true for monocots versus eudicots, as per the APG II system:

Flowers: In monocots, flowers are trimerous (number of flower parts in a whorl in threes) while in dicots the flowers are tetramerous or pentamerous (flower parts are in fours or fives).

Pollen: In monocots, pollen has one furrow or pore while dicots have three.

Seeds: In monocots, the embryo has one cotyledon while the embryo of the dicot has two.

Stems: In monocots, vascular bundles in the stem are scattered, in dicots arranged in a ring.

Roots: In monocots, roots are adventitious, while in dicots they develop from the radicle.

slice of onion, showing parallel veins

Leaves: In monocots, the major leaf veins are parallel, while in dicots they are reticulate.

However, these differences are not hard and fast: some monocots have characteristics more typical of dicots, and vice-versa. This is in part because "dicots" are a paraphyletic group with respect to monocots, and some dicots may be more closely related to monocots than to other dicots. In particular, several early-branching lineages of "dicots" share "monocot" characteristics, suggesting that these are not defining characters of monocots. When monocots are compared to eudicots, the differences are more concrete.

Taxonomy

The monocots are considered to form a monophyletic group arising early in the history of the flowering plants. The earliest fossils presumed to be monocot remains date from the early Cretaceous period.

Taxonomists have considerable latitude in naming this group, as the monocots are a group above the rank of family. Article 16 of the ICBN allows either a descriptive name or a name formed from the name of an included family.

Historically, the monocotyledons were named:
* Monocotyledoneae in the de Candolle system and the Engler system.
* Monocotyledones in the Bentham & Hooker system and the Wettstein system
* class Liliopsida in the Takhtajan system and the Cronquist system.
* subclass Liliidae in the Dahlgren system and the Thorne system (1992).
* clade monocots in the APG system and the APG II system. Each of the systems mentioned above use their own internal taxonomy for the group. The monocotyledons are famous as a group that is extremely stable in its outer borders (it is a well-defined, coherent group), while in its internal taxonomy is extremely unstable (historically no two authoritative systems have agreed with each other on how the monocotyledons are related to each other).

References and external links

* Chase MW, Soltis DE, Soltis PS, Rudall PJ, Fay MF, Hahn WJ, Sullivan S, Joseph J, Molvray M, Kores PJ, Givnish TJ, Sytsma KJ, Pires JC (2000). Higher-level systematics of the monocotyledons: An assessment of current knowledge and a new classification. In: Wilson KL, Morrison DA, eds. Monocots: Systematics and Evolution.. CSIRO, Melbourne. 3-16. ISBN 0643064370
*Tree of Life Web Project: Monocotyledons